A convenient, sensitive, and accurate blood or urine test for bone resorption is needed for rapid diagnoses and close monitoring of the effectiveness of treatments of metabolic bone diseases. Currently no such blood or urine test is generally available. Serum osteoclastic tartrate-resistant acid phosphatase (TrACP) has generally been recognized as a potential serum marker for bone resorption. The investigators have recently partially purified a hairy cell leukemia splenic TrACP which is thought to be similar to osteoclastic acid phosphatase. The investigators have also produced polyclonal antibodies and developed an ELISA assay against this enzyme. Preliminary application of this assay to patients with metabolic bone diseases gave results consistent with this assay performing as a bone resorption marker. Unfortunately, the investigators source of hairy cell leukemia spleens is now depleted. The investigators have located a new source for this enzyme, giant cell bone tumors (osteoclastomas), which have been shown to contain large amounts of TrACP and bona fide osteoclasts. Consequently, the investigators propose in this application to develop an immunoassay for osteoclastic TrACP from giant cell bone tumors. This could be a preferable source of TrACP because the TrACP from giant cell tumors may be more specific for osteoclastic TrACP than that from hairy cell leukemia spleens. The investigators' approaches are: (a) to purify an osteoclastic TrACP from giant cell bone tumors, (b) to determine whether the enzyme is a unique isoenzyme by determining the N-terminus and tryptic peptide amino acid sequences of the enzyme, (c) to produce monoclonal antibodies specific for osteoclastic TrACP, (d) to select a monoclonal antibody that best reflects osteoclastic TrACP level in serum, (e) to develop a specific and sensitive radioimmunoassay using the monoclonal antibody, (f) to standardize the assay, and (g) to validate the assay as a bone resorption index. The significance of this study is that it should allow the investigators to develop a sensitive and specific serum assay of bone resorption. Such an assay could be used for studies of the pathogenesis and treatment of metabolic bone diseases, especially osteoporosis. In addition, in future studies the monoclonal antibodies developed to osteoclastic TrACP could be employed to investigate the function of this enzyme. Finally, the sequence determined in this study will provide the means for future work on the molecular biology of this enzyme.